In some fields—automobiles and aerospace, for example—engineers have long taken care in their designs to both reduce unwanted emissions and minimize susceptibility to interference from other sources. But now, as designers pack more circuitry into smaller spaces, and the Internet of Things (IoT) adds connectivity to every device, electromagnetic compatibility (EMC) issues are assuming greater importance across a broad range of applications.
Depending on the frequency spectrum of the interference (broadband, pulse or transient, ESD, etc.) and the coupling mechanism (radiated or conducted), many different techniques are used to reduce EMC effects, including grounding, shielding, filtering and careful PCB layout.
For reducing conducted susceptibility (unwanted interference propagating via a wiring harness or power cable), stopping the noise at the point at which it enters the circuit—at the connector—is a very effective approach. A filter connector combines a standard connector with EMI/RFI suppression components to help solve EMC problems. As the filtering elements are contained within the connector itself, functional PCB area is kept to a minimum and weight is saved compared to a standard connector and discrete filtering components.
A filter connector saves space, offers design flexibility, reduces costs, and allows easy retrofit and quick upgrade of existing systems. Advantages of a filter connector also include:
- Low ground impedance: A ground plate and metallic shell provide a minimum impedance to ground and superior performance compared to a PCB-mounted filter.
- Elimination re-radiation: A filter connector at the interface leaves no path for noise to bypass the filter.
- Ground plane shielding: Filter connector ground planes shield the box, including at the connector port.
- Efficient space utilization: Filters located in the connector free up space on the PCB board.
- Consistent performance: Filter connectors provide more consistent pin-to-pin performance.
- Fewer components: And a reduced component count creates cost savings.
Construction of a Filter Connector
A filter connector includes an integrated filter by adding capacitors and ferrite inductors into the connector’s body. There are three types of capacitors commonly used: a ceramic chip capacitor as shown below; a tubular capacitor around the body of the connector pin; and planar capacitor arrays.
A planar capacitor array is a barium titanate ceramic disc or rectangle, which provides a common substrate for capacitance on each line of a connector. The capacitance values can be mixed including ground and feed-through lines. The typical range of capacitance available is 500 pF to 100,000 pF. The capacitor array also provides a continuous ground plane across the interface of the connector.
The ferrite inductor may either be in the form of a ferrite bead around the connector pin, or a ferrite slab through which all the pins pass. Ferrite beads are used to provide inductance of 0.5 uH to 5 uH, along with 10-100 ohms in the equivalent circuit.
Figure 1: “C” Filter connector construction. (Source: Amphenol)
Figure 1 shows the internal construction of a filter connector in the D-subminiature format. This connector contains a “C”-type filter, which is a simply a capacitor connected between the pin and connector ground.
Filter Selection
Selection of a particular filter circuit will depend on the required insertion loss characteristics and the system source and load impedances. A single-element type, such as a “C” or “L” filter, is the simplest solution; so-called C2 filters—with two parallel capacitors—are also common.
For more complex filtering tasks, multi-element filters, such as “PI” or “T” filters, can be used. Depending on the arrangement of components, a low-pass, high-pass, band-pass or band-stop characteristic is possible.
Figure 2 shows a comparison between different filter types and their applications.
Figure 2: Comparison of filter types. (Source: Amphenol)
More complex filters naturally lead to more complex filter connector construction. A more complex LC filter is shown below; this one is an ARINC connector for avionic use.
Figure 3: ARINC filter connector. (Source: Amphenol)
Factors Affecting Filter Connector Performance
Several operating factors affect the connector-filter performance and should be taken into account during the selection process:
- Operating voltage: The dielectric constant of a capacitor increases with applied DC voltage, resulting in a decrease in capacitance, and hence, filter performance. The magnitude of the change depends on the type of ceramic material used, the dielectric thickness and the DC voltage applied.
- Operating current: Increasing operating current causes magnetic saturation of inductive elements (ferrites), so filters with ferrite inductors (PI, LRC, CLR and T-types) will start to perform like “C” filters as the ferrite approaches saturation.
Operating Temperature Range: Capacitance and insertion loss performance are usually specified at a reference temperature of 25° C. The temperature coefficient of capacitance (TCC), specified either in terms of parts per million (ppm) per °C or a maximum percentage, describes the maximum change in capacitance value over a stated temperature range. Commonly used dielectrics have TCCs of +/-15 percent from -55° C to +125° C.
Filter Connectors for Transient Suppression
While conventional EMI filter connectors are effective in providing protection against low-energy transients, they offer little protection from high-voltage/high-energy transients that may result from lightning, load switching, electrostatic discharge (ESD) or electromagnetic pulse (EMP).
For those applications requiring protection of sensitive circuitry from such overvoltage events, Zener suppression diodes or metal oxide varistors (MOV) can be incorporated into the connector body either in combination with EMI filtering or alone.
An MOV is a non-linear, symmetrical, bipolar device. It conducts very little current at low-voltage levels, but once above the breakdown voltage, the voltage across the device remains fairly constant as it dissipates energy into a bulk metal oxide material. As a result, the varistor will effectively clamp both positive and negative high-current transients.
Combining the transient suppression device into the connector improves voltage clamping performance compared to a discrete solution by eliminating parasitic-lead resistance and inductance of board-level components.
AC-Line Filtering
One common requirement is to attenuate noise associated with AC line-powered equipment. In this case, EMI should be attenuated in both directions. Conducted emissions must be reduced to a level sufficient to pass regulatory limits such as FCC Part 15, which covers unintended emissions; and conducted susceptibility should be adequate to prevent incoming EMI from causing undesirable operating behavior.
Connectors with integrated AC-line filters are available specifically for this purpose. Often using the IEC form factor, the filter is an LRC type to attenuate both differential and common-mode EMI.
The Availability of Filter Connectors
Arrow Electronics offers more than 1,750 filter connector options, including circular, D-Subminiature, power and telecom connectors, as well as filtered terminal blocks.